According to prior art, optical quantum detectors (which absorb incident photons and generate electrical charge carriers) generally have highest sensitivity over a fairly small bandwidth, generally one octave or less. Contrary to the present disclosure, prior art detectors, used in optical imagers, generally make use of an optical anti-reflective coating to increase the amount of incident light that is coupled into their light-absorbing material.
Prior art infrared detectors are described in a Technical Information document (SD-12) by Hamamatsu Photonics K. K. Examples of prior photovoltaic and photoconducting detectors that have sensitivity in MWIR and/or LWIR wavelengths are described in an article by A. Rogalski (Journal of Applied Physics, vol. 93, no. 8, 15 Apr. 2003, pp. 4355-4391). In general, these detectors are formed as 2-dimensional arrays of detector pixels that are connected physically and electrically to a silicon read-out integrated circuit (ROIC). Photovoltaic detectors typically contain P-type semiconductor material, N-type semiconductor material and a PN junction. The incident light can be absorbed primarily in the P-type material, primarily in the N-type material or in substantially both P-type and N-type materials.
As depicted in FIGS. 1a-1d, prior infrared detectors 20, 22 and 24 often comprise a substrate 10 on which the array of detector pixels 12 is formed. The incident light 14 illuminates the substrate 10 and passes through the optically transparent substrate 10 to the detector pixels 12, as depicted in FIG. 1a. For these detectors, each detector pixel 12 may be connected to the ROIC 18 by means of a solder bump 16, depicted in FIG. 1b. With the detectors 20 and 22, the substrate 10 is generally not removed and individual pixels 12 are defined by etching mesa structures that include the PN junction of a detector pixel 12. For detector 24, illustrated in FIG. 1c, which comprise a thick film of the light-absorbing material 30, electrical vias 28 are etched through the absorbing film 30 and a PN junction 32 is formed around each of the vias 28. Metal 34 is then coated over the vias 28 and provide electrical connections between the PN junctions 32 and the ROIC 18. According to prior art, detectors 20, 22 and 24 can be coated with an anti-reflecting film to improve the capture of the light 14. The anti-reflecting film is normally composed of one or more quarter-wave thickness layers of material that have a value for its refractive index that is between the value of the refractive index of the incident medium (such as air) and the refractive index of the substrate 14.
According to prior art, the infrared detectors 20, 22 and 24 can achieve high external quantum efficiency only over a limited optical bandwidth because of their anti-reflective coatings. Because a quarter-wave thickness is achieved exactly for only one specific wavelength of the incident light, the anti-reflective coating is effective for only a small band of wavelengths (nominally less than an octave).
To achieve high internal quantum efficiency, the light-absorbing layer (or layers) of infrared detectors in the prior art must have a thickness that is sufficiently large to permit enough of the incident light, coupled in through its front surface, to be absorbed. In fact, the thickness of the absorber must be sufficient to absorb light at the longest wavelength of its desired band of operation. For high efficiency, that thickness is typically several times the value of the longest wavelength of the band, even when the detector has a metal reflector at its back side that enables the overall path-length of the light through the absorber to be doubled. Noisy “dark” current can be generated in the volume of the absorber because of thermal generation of electrical carriers. Thus, having a thick absorber means that the total volume of material contributing to the dark current is large, and the dark current is high. This degrades the detectivity of the detectors 20, 22 and 24. In contrast, a novel infrared detector (imager) presently disclosed, does not require an anti-reflecting film and it provides low reflection for incident light over a large bandwidth of multiple octaves with reduced dark current.
Like optical quantum detectors, solar cells have also been developed to absorb light, however at visible wavelengths. And, solar cells generally do not absorb light at MWIR wavelengths. Solar cells are generally made from material such as silicon. Although both solar cells and infrared imagers have been widely used commercial products for several decades, there does not appear to be any known attempts to combine the features of these two kinds of devices.
Surfaces with shallow pyramid-shaped features and the light trapping benefits of such surfaces are known from the field of solar cells. An article by M. A. Green, et al. (“Very High Efficiency Silicon Solar Cells—Science and Technology”, IEEE Transactions on Electron Devices, vol. 46, no. 13, October 1999, pp. 1940-1947) describes solar cells that contain pyramid-shaped surfaces. The light trapping properties of pyramidally textured surfaces is described in an article by P. Campbell and M. A. Green (Journal of Applied Physics, vol. 62, no. 1, 1 Jul. 1987, pp. 243-249). Prior art solar cell 38, depicted in FIG. 2, has pyramids 40 with height that are small compared to the overall thickness of the light-absorbing material 42. This is because for solar cells, the dark current noise is not a problem of concern. In contrast, in the infrared imager presently disclosed, the height of the pyramid is large compared to the overall thickness of the light absorbing material, with that pyramid height being about one half of the overall thickness of the light absorbing material.
Another prior art solar cell 44, depicted in FIG. 3, has pyramids 46 whose height is large compared to the overall thickness of the light absorbing material 48. This solar cell is described in an article by R. Brendel, et al. (“Ultrathin crystalline silicon solar cells on glass substrates,” Applied Physics Letters, vol. 70, no. 3, 20 Jan. 1997, pp. 390-392). In the solar cell 44, the PN junctions 50 are located near the sloped faces or sidewalls at the backside of the device. In contrast, for some embodiments of the present invention, the PN junctions may be located near the tips of the pyramid structures at the backside of the device. This allows the infrared imager presently disclosed to achieve reduced area of the junction depletion regions, thereby reducing the dark current from those depletion regions.
Another prior art solar cell with back-side electrical contacts whose PN junction area is small is described in an article by R. M. Swanson, et al. (“Point-contact silicon solar cells,” IEEE Transactions on Electron Devices, vol. ED-31, no. 5, May 1984, pp. 661-664). Solar cells having localized PN junctions at their back side as wells as pyramid-shaped texturing of their front side are depicted in FIG. 4 and are described in an article by R. A. Sinton, et al. (27.5 percent silicon concentrator solar cells,” IEEE Electron Device Letters, vol. EDL-7 no. 10, October 1986, pp. 567-569). Diffusion of the doped regions to create such PN junctions involves subjecting the material to fairly high temperatures, generally >400 degrees-C., and is not compatible with the processes involved in fabricating devices that have thin light absorbing material.
As depicted in FIG. 6, detectors, such as those in focal-plane array (FPA) imagers, have absorber regions that comprise a thick planar film that is solid (or continuous). It is described in more detail in an article by H. Yuan et al. (“FPA development: from InGaAs, InSb to HgCdTe”, Proceedings of SPIE Vol. 6940, paper 69403C, 2008). According to Yuan, each detector pixel is electrically connected, separately, to the read-out integrated circuit (ROIC) 56 by means of a solder bump 58. The array of detectors 64 also makes a common electrical connection to the ROIC 56 because they share a contiguous absorber layer 60 composed of n-HgCdTe material. The mesas that define the individual pixels of the detector array 64 are etched through the p+ layer 62 and only partly into the n-HgCdTe layer 60. The detector array 64 has volume of absorber material that contributes to the thermally generated (diffusion) dark current as defined by the thickness of that n-HgCdTe absorber layer 60 times the total area of the detector array 64. In this example, a way to reduce the diffusion-current component of the dark current would be to reduce the thickness of the absorber layer 60. However, such a reduction in layer thickness would also reduce the amount of incident light 66 that is absorbed, thereby reducing the quantum efficiency of the detectors.
A known method for reducing the volume of the absorber is shown in FIG. 7. This method involves placing the detector at the back end of an optical concentrator 70 such as a Winston cone. This method is more fully described in an article by T. Ashley, et al. (“Epitaxial InSb for elevated temperature operation of large IR focal plane arrays,” Proceedings of SPIE Vol. 5074 (2003), pp. 95-102). An array of such concentrators and detectors would have cones that abut each other at their entrances but those detectors would be physically isolated from each other. Thus, each detector would need to have both of its electrical connections (its P-connection and its N-connection) made to the detector itself. As a consequence, the ROIC would need to provide two electrical connections to each pixel rather than one electrical connection to each pixel, with the other electrical connection being a “common” connection.
According to FIG. 8, a prior-art solar cell 72 has a surface texture with a honeycomb 74 pattern. The solar cell 72 is described in more detail in an article by Zhao, et al. (“A 19.8% efficiency honeycomb multicrystalline silicon solar cell with improved light trapping,” IEEE Transactions on Electron Devices, vol. 46, no. 10, October 1999, pp. 1978-1983). The honeycomb 74 textured surface improves the trapping of the incident light so that the light can be absorbed by the solar cell 72 rather than being reflected away. The honeycombs 74 are a surface texture with a pitch or spacing of 14 μm and a thickness of less than 1 μm. The thickness of the absorber in the solar cell 72 is >200 μm. Thus, the volume represented by the honeycomb is a small fraction of the total volume of the light absorber. There is a need for a detector structure that has reduced volume of absorber material. There also is a need for a reduced-volume detector structure that provides low-resistance lateral flow of the photogenerated carriers, so that those carriers can be collected through the extractor regions or at the ohmic common contacts.
An article by Tokranova et al. (Proceedings of SPIE, Vol. 5723, pp. 183-189 (2005)) describes a solar cell comprising a film of porous silicon in which the 16 pores are filled with an organic material. The sunlight is absorbed primarily by the organic material, which in this case is copper phthalocyanine (CuPC), since the absorption efficiency of the porous silicon is poor. Absorption of the light in the CuPC results in generation of electrical charge carrying holes and electrons. The sides of the pores provide a large-area interface between the p-type CuPC and the n-type silicon material that serves to separate the photo-generated holes, which are transported in the porous silicon material, from the photo-generated electrons, which are transported in the CuPC. These prior art devices, however, would not be suitable for use as a low-noise detector. These devices have very large PN junction area and thus the relative contribution of the dark current due to generation in the junction depletion regions is very high. In contrast, the presently disclosed detectors have much smaller PN junction area that can benefit from the reduction of the absorber volume.
A novel infrared detector (imager) with low reflection for incident light over a large bandwidth of multiple octaves and with reduced dark current and reduced volume of absorber material is presently disclosed.
In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of every implementation nor relative dimensions of the depicted elements, and are not drawn to scale.